1. Field of the Invention
The present invention relates to an optical scanning apparatus used as an optical writing part of an image forming apparatus, and relates to an image forming apparatus and a color image forming apparatus including the optical scanning apparatus such as a laser printer, a digital compound machine, a laser facsimile, a laser plotter and the like.
2. Description of the Related Art
As an example of the optical writing part of the image forming apparatus, an optical scanning apparatus is known. The optical scanning apparatus includes a light source such as a semiconductor laser, an optical deflection part such as a polygon mirror, and a scanning optical system including a scanning lens and a mirror. In the optical scanning apparatus, an optical beam emitted from the light source is deflected by the optical deflection part, and the optical beam is concentrated on a scanned surface that is an image carrier via the scanning optical system so that the optical beam scans the scanned surface.
As to a color image forming apparatus, a tandem system is known. The color image forming apparatus of the tandem system includes a plurality of image carriers such as photosensitive drums arranged in parallel, and optical scanning apparatuses corresponding to each color. Each optical scanning apparatus scans the corresponding photosensitive drum by at least one optical beam corresponding to a color image signal so that a latent image is formed on each photosensitive drum. The latent images on each photosensitive drum are developed by toners of corresponding colors, so that toner images are superimposed with each other and are transferred to a transferring paper to obtain a color image. In the color image forming apparatus of the tandem system, image writing start positions of optical beams for each color should be the same. If the positions are shifted each other, displacements (shifts) of colors occur so that image quality deteriorates. Therefore, in the color image forming apparatus, a photodetector is provided in each optical scanning apparatus for detecting each optical beam at the scan start side. Thus, a time from a beam detecting time to a writing start time can be adjusted for each optical scanning apparatus so that the writing start positions can be aligned. That is, The adjustment of the writing start position is for the main scanning direction.
The above-mentioned color shifts may occur in the subscanning direction that is perpendicular to the main scanning direction. There are various causes for the color shifts in the subscanning direction. For example, the causes are a shift of an optical axis of the optical beam due to change of temperature, an eccentricity of the photosensitive drum and the like. Regardless of the causes, if a shift of an optical beam in the subscanning direction can be detected, the shift can be corrected by using a proper method.
Japanese laid-open patent application No. 10-235928 (document 1) discloses an example for solving the above-mentioned problem. The optical scanning apparatus in the Japanese laid-open patent application No. 10-235928 includes photodetectors each receiving a scan start optical beam as shown in FIGS. 2 and 4 of the document 1. By using the photodetector shaped as shown in FIG. 4, a shift amount in the subscanning direction can be calculated.
However, according to the photodetector of the document 1, there is a problem in that a time for a beam to pass through the photodetector cannot be detected accurately. For example, if a light amount of the optical beam changes, timing for detecting the beam by the photodetector may change.
In addition, according to the optical scanning apparatus of the document 1, the photodetectors are provided at one place that is an end of an image region. Thus, there is a problem in that the optical scanning apparatus cannot detect an inclination or a curve of the scanning line from the start to the end or in part of the scanning line, in which the inclination or the curve may occur when temperature varies in the main scanning direction or in the subscanning direction in each optical element. Thus, by the technique disclosed in the document 1, image quality deterioration due to the inclination or the curve of the scanning line cannot be corrected.
As mentioned above, the color image forming apparatus includes a plurality of optical scanning apparatuses. In each optical scanning apparatus, the semiconductor laser emits an optical beam. The optical beam is concentrated on the charged photosensitive drum via optical elements including the polygon mirror, lenses and the like, and the optical beam scans the photosensitive drum surface in the main scanning direction. Accordingly, image information is written into the rotating photosensitive drum by using optical beams having a predetermined pitch, and an electrostatic latent image corresponding to the image information is formed.
In the color image forming apparatus, temperature changes in each optical scanning apparatus due to heat generation by a polygon scanner and a fixing apparatus and the like. The change of the temperature may cause position shifts of lenses and the light source, cause change of a refractive index of an optical element and the like. Accordingly, optical characteristics in the optical scanning apparatus may change so that a position shift of an optical beam spot or a curve of the scanning line may occur on the scanned surface. As a result, relative positions of the scanning lines of each color are shifted so that the color shifts to deteriorate color image quality occur. The causes of the color shifts are not only the temperature change but also dimension errors in mechanical parts. In addition, there are various causes other than these causes.
As to the optical scanning apparatus, for increasing a speed for recording an image, it is necessary to rotate the polygon mirror at higher speed. However, the rotation speed of the polygon mirror is limited due to vibration and noise. Thus, instead of increasing the rotation speed of the polygon mirror, various methods have been proposed in which a plurality of lines are recorded at a time by using a plurality of optical beams.
The optical scanning apparatus adopting the method for using a plurality of optical beams at a time includes an light source unit including a semiconductor laser array (LDA) having multiple light-emitting points or a plurality of semiconductor lasers (LD) each having one light-emitting point. In the light source unit, a plurality of optical beams are integrated to form beam spots in the subscanning direction. The multiple optical beams emitted from the light source unit are deflected by the polygon mirror to scan a photosensitive drum so that multiple lines with a predetermined pitch are recorded at a time. However, although the light source unit is initially adjusted such that a predetermined subscanning direction pitch is obtained, the pitch is changed from the predetermined pitch due to external factors such as vibration, temperature and the like. Therefore, it is necessary to detect the shift of the subscanning direction pitch to correct it for obtaining a high quality image.
Some methods have been proposed for detecting and correcting the shift.
Japanese laid-open patent application No. 7-72399 (document 2) discloses an example of the method. In a system of the document 2, sensors are used for optical beam position detection in which sides of a scan start end of the sensors are not parallel to each other. In the system of the document 2, a time interval for passing through the start ends of the sensors are measured for each beam individually, and a difference between time intervals is converted to the subscanning direction pitch between beams. However, according to this method, detection positions may be changed due to power change of the optical beam or sensitivity change of the sensor caused by change of temperature or power source voltage. Thus, there is a problem in that the measured values of scanning positions or pitches vary.
Japanese laid-open patent application No. 9-325288 (document 3) discloses another example. In a scanning apparatus in document 3, a plurality of photosensors are arranged in a main scanning direction in which the photosensors include a photosensor that is parallel to the subscanning direction and a photosensor that is not parallel to the subscanning direction. The pitch is obtained by using time intervals of pulses generated when optical beams pass through the photosensors. In the method of document 3, output signals of the photosensors are added for detecting the beam positions. However, for performing such calculation, a circuit for detecting a 0 cross point in addition to the circuit for adding the signals is necessary. Therefore, there is a problem in that a large circuit size is necessary.